Silicone release coatings are very, thinly applied coatings of silicone polymers. Typically these coatings are very thinly applied at levels of 1 g/m2 at very high speeds frequently ranging at or above 300 m/min., across paper or film liners that may be as wide as 2 m. Coating imperfections such as pinholes, areas of incomplete coverage, and variations in coating weight may occur in such a high speed operation, even when using sophisticated offset gravure and multi-roll film splitting techniques. The high speed of many coating processes thus can generate a large quantity of poorly coated and therefore unacceptable product in a very short time. Even when the silicone coating is fully cured, such imperfections lead to a coated product that fails to perform as intended.
It therefore is desirable to be able to assess the quality of the silicone coating being applied, as it is applied, i.e. on-line. One possible method of assessing coating quality would be to dissolve spectrophotometrically active fluorescent dyes in the silicone release coating. Since this is possible, then a fluorescence detector downstream of the coating apparatus can spectrophotometrically scan the newly coated product as it is excited by ultraviolet light of the appropriate frequency in a fashion where the fluorescent response of the coating indicates the amount of the coating applied per area of substrate coated. Ultraviolet laser excitation is particularly well suited to such an application.
Since all organic molecules, and particularly silicones, are active in the infrared region of the spectrum, infrared spectral analysis of a coating would be a particularly complicated means of assessing coating quality. Visible dyes would present aesthetic problems that might or might not be undesirable to the end user of the product. Thus by a process of elimination, fluorescence detection of a dye marked coating becomes the method of choice for the purpose of assessing coating quality.
In order to be useful as a fluorescent dye marker an organic compound must have an ultraviolet absorption spectrum that has a fairly high molar extinction coefficient so that it is not necessary to incorporate large quantities of the dye molecule in the formulation. The organic dye marker compound must also be strongly fluorescent, upon excitation at the proper ultraviolet wavelength. This is desirable for two reasons, first large quantities of an additive may impart undesirable performance characteristics to the formulation, and second, most fluorescent ultraviolet dye marker compounds are insoluble in silicones. Such marker compounds are typically crystalline solids possessing a high melting point. These properties are associated with extended molecular structures that possess a great deal of conjugated olefinic bonds, a feature necessary to impart ultraviolet fluorescent activity. The chemical structure that render these compounds spectrophotometrically active in the ultraviolet region of the electromagnetic spectrum also make them polar. This polarity generally leads to poor solubility in polymeric silicone formulations.
The poor solubility of fluorescent dye marker compounds in silicone compositions has been previously solved by modifying the silicone polymer such as the epoxysilicone-polyether block copolymers described in U.S. Pat. Nos. 5,227,410 and 5,240,971. This approach has not been fully satisfactory. Modifying the silicone molecule to improve its compatibility with polar fluorescent dyes tends to change the cure response of the silicone so modified and usually causes a deterioration in the release characteristics of the cured silicone.
Fluorescent active photo-curable silicones are known in the art. U.S. Pat. No. 4,978,731 discloses the incorporation of ultraviolet fluorescent substituents via the hydrosilation of a silyl hydride with an ultraviolet fluorescence active olefinic molecule that becomes an ultraviolet fluorescence active substituent on a silicone polymer: ##STR1## The ultraviolet fluorescent active substituted SiH-containing silicone polymer is then subsequently reacted with a glycidyl ether. Because the glycidyl ether contains epoxy groups, the fluorescent active silicone polymer becomes an ultraviolet fluorescent silicone that is epoxy functionalized as well as photocurable via cationic photocure processes.
It is known to functionalize amino-functional silicone polymers by reacting acid chloride fluorescent materials such as dansyl chloride with the amino group of the silicone polymer: ##STR2## It is also known to functionalize halo-alkyl substituted silicone polymers by reacting hydroxy coumarins as follows: ##STR3## These reactions are disclosed in U.S. Pat. Nos. 5,118,776; 5,176,906; 5,302,371; and 5,107,008 to Ziemelis et al. The silicones of Ziemelis et al. also include a vinyl/silyl hydride functionality that allows their cure via platinum catalyzed hydrosilation, however it is to be particularly noted that none of these materials is photocurable.